1. Field of the Invention
The present invention relates to a reader and more particularly, to a reader having a high-resolution mode and a high-speed mode using a CCD sensor.
2. Description of the Related Art
Recently, as means for inputting a document and drawing data or input means for digital copiers and facsimile, there has been widely used a reader using a CCD sensor.
A CCD sensor used in this reader will be described by referring to FIG. 5.
Analog shift registers arranged in odd rows and analog shift register arranged in even rows are finally combined into an output signal CCD.sub.OUT through buffers. Symbols S.sub.1, S.sub.2, . . . S.sub.N denote light receiving means. OSR.sub.1, OSR.sub.2, . . . OSR.sub.N/2 denote odd-row shift registers for shifting out the analog outputs of light receiving means on the odd side (light receiving means arranged in odd rows) an ESR.sub.1, ESR.sub.2, . . . ESR.sub.N/2 denote even-row shift registers for shifting out the analog outputs of light receiving means on the even side (light receiving means arranged in even rows). Furthermore, symbol SH denote a start pulse for the shift operation of the shift registers, symbols .phi..sub.1 and .phi..sub.2 denote transfer pulses, and symbol .phi..sub.R denotes a reset pulse.
Then, referring to FIG. 3, the positional relationship between a line-shaped fluorescent lamp 33 and a CCD sensor 37 in a reader will be described.
A manuscript 31 is laid on a glass table 32. In FIG. 3, the glass table 32 and the manuscript 31 are spaced from each other but actually are closely in contact with each other. The manuscript 31 is illuminated through the glass table 32 with a ray of light emitted from the fluorescent lamp 33 with the longitudinal direction placed vertically to the surface of a paper. The ray of light reflected from the manuscript passes again through the glass table and is reflected from the mirror 35. The ray of light reflected from the mirror 35 is focussed at a lens 36 and illuminated to the light receiving surface of the CCD sensor 37. Numeral 34 denotes an optical unit including the mirror 35, the lens 36 and the CCD sensor 37, while numerals 38 and 39 denote a pulse motor and a cabinet, respectively.
As shown in FIG. 4, a conventional reader incorporates a CCD sensor 40 for converting a received light into an electrical signal and the CCD sensor 40 is connected via a capacitor C to an input of a buffer 41. The input of the buffer 41 is connected to a 5 V electric power source via an analog switch. The output of the buffer 41 is connected to the input of an AD converter 42. The outputs D.sub.0 -D.sub.7 of the AD converter 42 are connected to a control circuit 43. The output FLON of the control circuit 43 is connected to the input of a lighting device 48. The output of the lighting device 48 is connected to the input of a fluorescent lamp 33. When the FLON turns to "1", the fluorescent lamp 33 turns on. The outputs T.sub.STEP and FORWARD of the control circuit 43 are connected to the input of a pulse motor drive circuit 46. The input of the pulse motor drive circuit 46 is connected to the input of a pulse motor 38. If "FORWARD" is "1" the pulse motor 38 is rotated by the pulse motor drive circuit 46 so as to forward the optical unit 34 and the fluorescent lamp 33 by one step, when the pulse motor receives one time of T.sub.STEP pulse. Similarly, if "FORWARD" IS "0", the pulse motor 38 is rotated so as to reverse the optical unit 34 and the fluorescent lamp 33, when the pulse motor receives the T.sub.STEP pulse.
The control circuit 43 comprises an oscillator 44 and a clock generating circuit 45 connected thereto. A basic clock t.sub.0 is oscillated by the oscillator 44 and divided in frequency by the clock generating circuit 45 to generate various control pulses SH, .phi..sub.1, .phi..sub.2, .phi..sub.R and T.sub.CLAMP. Control pulses SH, .phi..sub.1, .phi..sub.2 and .phi..sub.R are supplied to the CCD sensor 40 and a control pulse T.sub.CLAMP is supplied to the analog switch.
FIG. 2 shows drive signals SH, .phi..sub.1, .phi..sub.2, .phi..sub.R to the CCD sensor 40, a CCD output CCD.sub.out a clamp signal T.sub.CLAMP for clamping the CCD.sub.out a timing of ADIN which is an input signal to the AD converter 42.
Then, the operation of an analog switch and C shown in FIG. 4 will be described.
Ordinarily, the CCD output CCD.sub.out is not defined as a DC signal, for example, outputted in the negative direction from a standard voltage of 4 V, as shown in FIG. 2. This standard varies between 3.5 V and 4.5 V according to each CCD sensors. However, since the AD convertor will digitalize an analog voltage of 5 V or less, it is necessary to shift the DC level in such a manner that a value of output voltage from the capacitor C and the analog switch may be 5 V or less. Thus, the capacitor C comes to be always charged to 1 V.
Here, the CCD.sub.out has a load of long transmission path and is usually amplified in use, and accordingly noises in the relevant amplifier are randomly superimposed onto the CCD.sub.out. For example, the DC level of the CCD.sub.out is normally 4 V but is assumed to become 3.8 V due to the random noises mentioned above. At this time, C has to be rapidly charged to 1.2 V that is 0.2 V higher than a normal 1 V. Thus, setting the clamp time to t=300 nsec and the ON resistance of the analog switch to 50 .OMEGA., The capacitance C becomes as follows:
C.OMEGA.R&lt;&lt;t PA1 C.ltoreq.300 nsec/50 .OMEGA.=6000 pF.
In conventional scanners, a value of 1000 pF or less is selected. By secure clamping of the DC level of each individual CCD signal outputs in this way, an image with reduced random noise is obtained as a reader.
On the other hand, Japanese Patent Application Laying Open No. 5-48460 discloses a three-dimensional integrated circuit comprising an AD converter for executing a digital to analog conversion with high-speed and high-resolution.
However, in a reader described by referring to FIGS. 2 to 5, through an image with reduced random noise is obtained as a reader, by secure clamping of the DC level of each individual CCD signal outputs, the stable period of the CCD.sub.out is further needed before and after the clamp signal T.sub.CLAMP, thereby the operating speed becomes slower. In addition, a three-dimensional integrated circuit disclosed in Japanese Patent Application Laying Open No. 5-48460 comprises a complicated circuit and becomes slower in the rate of AD conversion than ordinary AD converter.
In order to solve these problems, as shown in Japanese Patent application Laying Open No. 6-253091, the applicant of the present invention proposes a reader comprising: a CCD sensor for individually outputting an output of the light receiving section arranged in odd rows and an output of that arranged in even rows; a first AD converter corresponding to either one of the outputs of odd rows or even rows; a second AD convertor corresponding to the other of the outputs of odd rows or even rows; and means for supplying the other of the outputs of odd rows or even rows to the first AD convertor when a high-resolution mode is selected.
However, in the reader of Japanese Patent Application Laying Open No. 6-253091, the quality in the high-resolution mode is equal to the quality of ordinary reader though the device becomes complicated.